1. Field of the Invention
This invention relates to engine fuel control systems which incorporate an air/fuel ratio feedback control.
2. Prior Art
Various fuel control systems are known in the prior art in which the quantity of fuel fed to the engine is controlled by sensors in the exhaust gas which give an indication of the air fuel ratio. Nevertheless, it remains extremely difficult to compensate for the ever changing operating conditions of the engine, the variations among different engines and so on as to always operate the engine with a predetermined air fuel ratio. This drawback may become critical when the engine is equipped with a catalytic converter for reducing undesirable components of the exhaust gases.
A widely used technique to control the air fuel ratio in stoichiometric feedback controlled fuel metering systems is limit cycle integral control. In this technique, there is a constant movement of a fuel metering component in a direction that always tends to counter the instantaneous air fuel ratio indication given by a typical two state exhaust gas oxygen (EGO) sensor. For example, every time an EGO sensor indicates a switch from a rich to a lean air fuel ratio mode of operation, the direction of motion of a typical carburetor's metering rod reverses to create a richer air fuel ratio condition until the sensor indicates a change from a lean to rich air fuel ratio condition. Then, the direction of motion of the metering rod is reversed again this time to achieve a leaner air fuel ratio condition.
Referring to FIGS. 1a and 1b, step like changes in the sensor output voltage initiate ramp like changes in the actuator control voltage. When using the limit cycle integral control, the desired air fuel ratio can only be attained on an average basis since the actual air fuel ratio is made to fluctuate in a controlled manner about the average value. The limit cycle integral control system can be characterized as a two state controller with the mode of operation being either rich or lean. The average deviation from the desired value is a strong function of a parameter called engine transport delay time, tau. This is defined as the time it takes for a change in air fuel ratio, implemented at the fuel metering mechanism, to be recognized at the EGO sensor, after the change has taken place.
The engine transport delay time is a function of the fuel metering system's design, engine speed, air flow, and EGO sensor characteristics. Because of this delay time, a control system using a limit cycle technique always varies the air fuel ratio about a mean value in a cyclical manner, a rich air fuel ratio time regime typically followed by a lean air fuel ratio time regime. The shorter the transport delay time is, the higher will be the frequency of rich to lean and lean to rich air fuel ratio fluctuation and the smaller will be the amplitudes of the air fuel ratio overshoots. It can be appreciated that a system with no engine transport delay time is the ideal.
In internal combustion engines having a catalytic converter, such as a platinum rhodium converter, it is often desirable to operate at stoichiometry in order to minimize emissions. At stoichiometry, the air fuel ratio is 14.64. In such a system the engine base fuel mass flow is calculated by measuring air mass flow and dividing by 14.64. Further, internal combustion engines having such air fuel ratio control are often capable of operating in both open and closed loop modes. In the closed loop mode, an exhaust gas oxygen sensor senses the air fuel ratio and corrects the base air fuel control signal. In the open loop mode, the air fuel ratio is established as a function of stored operating parameters in view of measured air flow. However, such stored operating parameters and measured air flow may not reflect engine wear and history. For example, it may be desirable to compensate engine open loop air fuel ratio control for effects caused by uncalibrated air leaks and fuel system aging. Typically, open loop operations occur when there is cold engine operation and wide open throttle engine operation. Under such conditions the EGO sensor response is not sufficient for adequate control. Fuel control is obtained normally by detecting the air mass entering the engine. Since the exhaust gas oxygen sensor is out of the control loop, this operation is referred to as being open loop. However, uncalibrated air leaks and fuel system aging can cause difficulty in achieving a desired air fuel ratio during open loop operation.
Further, initial installation and calibration of airmeters on vehicles has indicated that there is an additive or offset error between bench and vehicle calibrations at idle. This error can be of the order of 30%. since the estimated injector error at idle is approximately 5%, the probable cause of this error is air leakage into the engine downstream of the airmeter. This error is greatest at idle when airflow is at a minimum and manifold pressure is low. Air leakage of this nature has been a problem in airmeter controlled systems, usually requiring individual vehicle calibrations to eliminate the problem. This represents an undesirable complexity and expense. These are some of the problems this invention overcomes.